Understanding Bayesian Optimisation graph
The original graph for the aforementioned Bayesian Optimisation is similar to the graph in these slides (slide 18) along with the calculations.

So, according to the tutorial the graph shown should actually have the term $p(D|m)$ on the y-axis, thus making it a generative model.Now the graph starts to make sense, since a model with low complexity cannot produce very complex datasets and will be centred around 0, while very complex models can produce richer datasets which makes them assign probability thinly over all the datatsets (to keep $\sum_{D'}p(D'|m) = 1$).